The invention relates to an inductive path sensor, for example a valve sensor, for determining the position of an influencing element, for example a valve rod or piston, and a process for determining the position of an influencing element with an inductive path sensor.
Path sensors for determining the position of an influencing element, which are often also called position sensors, are known in a host of embodiments and for a host of applications. These path sensors can on the one hand be classified according to whether the motion of the influencing element to be monitored is primarily linear motion, thus a distance is to be acquired by the path sensor, or the motion of the influencing element is first of all circular motion, so that the path sensor monitors or establishes the angle of rotation of the influencing element. Path sensors which detect an angle of rotation are often also called angular resolvers.
In addition, path sensors can also be classified according to their physical operating principle. For example, inductive, capacitive or optoelectronic path sensors are known.
The subject matter of this invention is an inductive path sensor, especially one with which linear motion of an influencing element, for example a distance, can be measured. These known inductive path sensors have several coils, of which at least one coil is made as the primary coil and at least another coil is made as a secondary coil. The coils are generally built according to the transformer principle, so that one secondary coil is located laterally adjacent to one primary coil at a time. The inductive coupling between the middle primary coil and the two laterally arranged secondary coils is changed by the position of the influencing element which is located in the area of the cylinder axis of the cylindrical coil system and is made for example as a magnetically conductive rod. These inductive path sensors are known from DE 43 37 208 A1 and DE 19632 211 A1.
But in the known inductive path sensors it is a disadvantage that on the one hand the structural length of the path sensor is distinctly longer than the maximum distance of the influencing element which can be monitored, so that at a give path length to be monitored a path sensor up to 100% longer is required. This is undesirable especially where only a limited installation space is available. On the other hand, in the known inductive path sensors the attainable measurement accuracy is often not sufficient or it can only be increased by increased circuitry cost.
Thus, the object of this invention is to make available an inductive path sensor which has a structural length as small as possible and in addition enables measurement accuracy as high as possible. In addition, the object of this invention is also to devise a process with which the position of the influencing element can be precisely and reliably acquired within a housing by means of an inductive path sensor.
The aforementioned object is achieved as claimed in the invention by an inductive path sensor with several coils arranged in succession, with at least one capacitor, with at least one amplifier element, with at least one changeover switch and with an evaluation unit, one coil at a time and the capacitor or a capacitor forming an oscillating circuit, and one oscillating circuit and the amplifier element or an amplifier element forming an oscillator, the individual coils and the individual oscillators being chosen in succession by the changeover switch and the evaluation unit measuring the change of impedance of the coil chosen by the changeover switch or of the oscillating circuit chosen by the changeover switch depending on the position of the influencing element relative to the respective coil.
By using several successive coils, the coils being located in succession in the direction of the position of the influencing element to be ascertained, and the evaluation unit measuring the change of impedance of each coil and each oscillating circuit in succession depending on the position of the influencing element by the changeover switch, an inductive path sensor can be implemented with a structural length which is only slightly greater than the total length of the distance to be monitored.
It was stated above that the evaluation unit measures the change of the impedance of each coil or each oscillating circuit. Preferably the evaluation unit measures the change of the frequency of each coil or each oscillating circuit depending on the position of the influencing element. But in addition it is also possible for the evaluation unit to measure the change of the inductance of the coil or of the oscillating circuit or the change of the amplitude of the oscillating circuit as a function of the position of the influencing element.
If according to the preferred embodiment of the invention the evaluation unit measures the change of frequency, generally the change of frequency of the oscillating circuit is measured depending on the position of the influencing element. But at least theoretically it is also possible for the change of the frequency to be measured solely by the coil, inasmuch as each actual coil in addition to the primarily characteristic inductance also has an ohmic resistance and several parasitic capacitances. Thus the actual coil has a natural resonant frequency which is determined by the inductance and the parasitic capacitances of the coil. But generally the change of frequency of the oscillating circuit consisting of a coil and an additional capacitor is measured by the evaluation unit.
According to one preferred embodiment of the invention, each oscillating circuit has the same, especially the identical capacitor, and each oscillator has the same, especially the identical amplifier element. In other words, the inductive path sensor does have several successive coils, but only one capacitor and also only one amplifier element, for example an operational amplifier. The individual oscillating circuits thus each consist of the same capacitor and a coil chosen by the changeover switch. This has first of all the advantage that only a few components are required for the inductive path sensor, by which it can be built both economically and also with a further reduced structural size. In addition, when the individual oscillators are each composed of the same capacitor and the same amplifier element, measurement signal adulterations due to component variations do not occur, as would be the case when using several capacitors or several amplifier elements.
If it was stated above that the inductive path sensor according to one preferred embodiment has only one capacitor and only one amplifier element, it is of course only meant that the individual coils are always interconnected via the changeover switch to the same capacitor or to the same amplifier element, by which a number of oscillators corresponding to the number of coils is accomplished. Of course it is also possible for the amplifier element to be implemented in terms of circuitry by two or more amplifier elements, for example by two operational amplifiers. Likewise the capacitor of the oscillating circuit can be implemented in terms of circuitry by several capacitors.
Influencing the coil or the oscillating circuit depending on the position of the influencing element is theoretically based on three different physical effects which differ greatly in their action, depending on what type of influencing element is used.
Within the framework of this invention the influencing of the impedance of the oscillating circuit by the influencing element based on the transformer principle is preferably evaluated. The physical effect called the transformer principle here is based on that coil of the oscillating circuit generating an alternating electromagnetic field which induces a voltage in the adjacent bodyxe2x80x94the influencing elementxe2x80x94first of all according to Faraday""s Law. When using an influencing element of a material with relatively high conductivity the induced voltage leads to current flow in the influencing element. This current resulting from the xe2x80x9csecondaryxe2x80x9d voltage induced in the influencing element results for its part in an alternating electromagnetic field which is opposite the xe2x80x9cprimaryxe2x80x9d alternating electromagnetic field, i.e. opposite the alternating electromagnetic field generated by the coil. This opposite xe2x80x9csecondaryxe2x80x9d alternating electromagnetic field causes a reduction of the inductance and thus an increase of the frequency of the oscillating circuit. Preferably this frequency increase is measured and evaluated depending on the position of the influencing element by the evaluation unit.
The second physical effect which occurs when the impedance of the oscillating circuit is influenced by the influencing element is the influencing of the magnetic resistance of the magnetic circuit. If there is no influencing element in the vicinity of the coil, the magnetic resistance is determined solely by the air and is thus very large. If there is an influencing element of a preferably ferromagnetic material in the vicinity of the coil, in this way the electromagnetic resistance of the magnetic circuit is reduced; this can be established on the reduction of the frequency of the oscillating circuit.
The third physical effect which occurs when the impedance of the oscillating circuit is influenced by the influencing element is the xe2x80x9cgenuinexe2x80x9d damping of the oscillating circuit by withdrawing energy from the alternating electromagnetic field of the oscillating circuit as a result of eddy current losses in the influencing element. This physical effect which is called xe2x80x9cgenuinexe2x80x9d damping here is evaluated generally in inductive proximity switches.
Since theoretically all three effects are active, provisions must be made for the two effects which are not to be used for evaluation to be negligibly small compared to the effect which is to be used for evaluation.
If the transformer effect is used for evaluation, as is preferable, this transformer effect should not be counteracted by the fact that the resistance of the magnetic circuit and thus the frequency are reduced by the ferromagnetic material. Preferably the influencing is evaluated based on the transformer principle, because in this way it can be guaranteed by the suitable choice of the frequency that the measurement results are essentially independent of the material of the influencing element used. The ferromagnetic effect can remain ignored. The frequency of the uninfluenced oscillating circuit to be chosen for this purpose is preferably above 500 kHz, for example between 500 kHz and 10 MHz.
With regard to the manner in which a change of the impedance of the oscillating circuit chosen by the changeover switch is measured and evaluated by the evaluation unit, there are several different possibilities known to one skilled in the art which differ among others in the complexity of the evaluation unit. Generally the evaluation unit has at least one microprocessor in which the measured values can be processed, converted and optionally stored in an additional memory.
If according to the preferred embodiment of the invention the change of frequency of the influencing element is measured by the oscillating circuit, the inductive path sensor as claimed in the invention advantageously has at least one counter which on the one hand is connected to the oscillator and on the other hand to the evaluation unit. According to a first preferred embodiment, the counter counts the number of oscillations until a preset value is reached and the evaluation unit measures the time which passes until the counter has reached this preset value. Here it is especially advantageous that time measurement with the evaluation unit, for example a microprocessor, can be accomplished very easily. If the transformer principle is used so that the presence of the influencing element in front of the selected coil increases the frequency of the oscillating circuit, in the above described type of evaluation this is established by the fact that the counter reaches the preset value faster, compared to the state in which the coil and thus the oscillating circuit are not influenced by the influencing element. The evaluation unit thus measures a time which is shorter compared to the uninfluenced state.
In one alternative embodiment the counter counts the number of oscillations of the oscillator during a given time interval and this number is evaluated by the evaluation unit.
According to another advantageous embodiment of the invention the inductive path sensor has a second counter which is connected on the one hand to the first counter and on the other to the evaluation unit and the changeover switch. The second counter generates addresses, preferably in dual code, which are increased by one each time, starting with one, when the first counter has reached the preset value or has counted the given time interval. Because the second counter is connected to the changeover switch, the changeover switch is switched by the address produced by the second counter and thus the next coil is selected. By connecting the two counters in succession the changeover switch is switched and the next coil is selected automatically at the correct and at the same time fastest possible instant, by which the reaction speed of the inductive path sensor is increased.
For the inductive path sensor as claimed in the invention, basically any number of different types of coils can be used. With respect to component cost and evaluations it is however practicable if for example 8, 16 or 32 coils are used, which are selected accordingly with one, two or four 1 out of 8 multiplexers. These 1 out of 8 multiplexers are commercially common and are therefore available relatively economically.
The individual coils are each made preferably identically and can be formed for example as hollow cylindrical coils or as flat coils. The individual coils are also preferably arranged symmetrically to one another. If the coils are made as hollow cylindrical coils, the influencing element is located movably along the cylinder axis of the coils, for example within a tube. The influencing element is then preferably likewise made cylindrically, for example disk-shaped, or rod-shaped.
According to a last advantageous embodiment of the inductive path sensor as claimed in the invention which is to be briefly explained here, the individual coils are made or arranged such that they are decoupled among one another.
If the individual coils are located relatively close to one another, which is generally the case as a result of the desired short structural length of the inductive path sensor, there is relatively great electromagnetic coupling which has an adverse effect on the measurement result between adjacent coils. The coil which has not been selected by the changeover switch according to the preferred embodiment is not connected to a separate capacitor so that this coil has only its parasitic capacitances which were mentioned initially. Since these parasitic capacitances are much smaller than the capacitance of the capacitor of the oscillating circuit, a coil which has not been selected has a higher natural resonant frequency. Due to the greater ratio of inductance to capacitance the coil which was not selected also has a clearly higher quality than the coil which was just selected by the changeover switch and which is connected to the capacitor of the oscillating circuit. In a small frequency range this leads suddenly to frequency guidance of the oscillator by the coil which was not selected, by which the frequency of the oscillator is detuned and thus the measurement signal erroneously reproduces the actual position of the influencing element.
To prevent or attenuate this unwanted coupling between adjacent coils, there are now various possibilities for decoupling the individual coils among one another. The first preferred embodiment of the inductive path sensor as claimed in the invention consists in that the individual coils are each located in a pot-type core, the pot-type cores being open toward the influencing element. The use of pot-type cores which are known for example in inductive proximity sensors and which consist of a material with a permeability as high as possible is especially suited when hollow cylindrical coils are used.
The use of these pot-type cores in addition to the primarily desired decoupling of the adjacent coils advantageously also causes alignment of the electromagnetic field in the direction of the cylinder axis of the coils and thus in the direction of an influencing element located in front of the coil. In this way the available useful signal is increased. In addition, shielding of the inductive path sensor to the environment takes place by using pot-type cores so that on the one hand no electromagnetic fields are emitted into the vicinity by the inductive path sensor, on the other hand interference of the measurement signal by articles located in the vicinity of the inductive path sensor, especially metal articles, is prevented.
Alternatively to the use of pot-type cores, the individual cores can also be decoupled among one another by the coils being located on a flexible carrier of a ferrite-polymer composite, by the corresponding folding of the flexible carrier or parts of the flexible carrier the individual coils being shielded against one another. In addition or besides, it is also possible to separate the individual coils from one another by short-circuiting rings.
The initially mentioned object is achieved in a process for determining the position of the influencing element with an inductive path sensor with several successive coils, with at least one capacitor, with at least one amplifier element, with at least one changeover switch and an evaluation unit first of all and essentially by the process having the following steps:
Selection of a coil or an oscillator by the changeover switch,
Measurement of the impedance of the coil selected by the changeover switch and of the oscillating circuit selected by the changeover switch by the evaluation unit as a function of the position of the influencing element relative to the coil, the aforementioned steps being repeated until the impedance of all coils has been measured by the evaluation unit.
This process can preferably be carried out with the above described inductive path sensor.
Preferably the process as claimed in the invention for determining the position of the influencing element, for example of a piston, is carried out with an inductive path sensor which has a first counter and second counter. The first counter is on the one hand connected to the oscillator and on the other to the evaluation unit and the second counter is connected on the one hand to the first counter and on the other to the evaluation unit and the changeover switch.
The first counter which counts the number of oscillations of the just selected oscillator, when a preset value is reached, delivers a pulse to the second counter, the second counter producing addresses which are increased by one each time, starting with one, when the second counter receives the pulse from the first counter. The address which is preferably produced in dual code causes first the commutation of the changeover switch. In addition, the evaluation unit receives from the address produced by the second counter on the one hand the information which coil has been selected by the changeover switch, on the other hand the information how long the respective address is present, and thus the information how long the first counter needed to reach the preset value when counting the oscillations of the oscillator. This time interval, as already cited, is a measure for influencing the frequency of the oscillating circuit and thus a measure for the position of the influencing element.
The time interval for which the respective address is present can be determined especially easily by the evaluation unit by the evaluation unit measuring the time between the high/low flanks at the lowest order output of the second counter. The address of the second counter which is present in dual code is thus used as the gate time for the evaluation unit.
According to a last advantageous embodiment of the process as claimed in the invention which is to be briefly described here, in the calibration process the influencing element is moved over the maximum measurable length of the inductive path sensor and the values obtained during the calibration process are stored as correction and reference values in the evaluation unit and in an additional storage. In this way it is possible to use different influencing elements with different dimensions or of different materials. Component tolerances of the inductive path sensor or changes as a result of temperature fluctuations can be compensated by this calibration process.